Thin film Si:O alloys have been grown by plasma enhanced chemical vapor deposition, as intrinsic or highly doped (1 to 5 at. % of B or P dopant) layers. UV-visible/near-infrared spectroscopy revealed a great dependence of the absorption coefficient and of the optical gap (E-g) on the dopant type and concentration, as E-g decreases from 2.1 to 1.9 eV, for the intrinsic or highly p-doped sample, respectively. Thermal annealing up to 400 degrees C induces a huge H out-diffusion which causes a dramatic absorption increase and a reduction of E-g, down to less than 1.8 eV. A prototypal solar cell has been fabricated using a 400 nm thick, p-i-n structure made of Si:O alloy embedded within flat transparent conductive oxides. Preliminary electrical analyses show a photovoltaic (PV) effect with an open circuit voltage of 0.75 V and a spectral conversion efficiency blue-shifted in comparison to a-Si:H based cell, as expected since the higher E-g in Si:O alloy. These data are presented and discussed, suggesting Si:O alloy as promising material for PV device fabrication.